Sinter bonded containment tube

ABSTRACT

A containment tube includes a sealed tube comprising silicon carbide, first and second ends, an inner bore extending along at least a portion of its axial length between the first and second ends, and contains a radioactive material within the bore of the sealed tube. The first end has a plug residing in the inner bore to close the first end, and the second end has a distal wall that closes the inner bore at the second. At least one of the first or second ends is bonded to the sealed tube by a sinter bond.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of and claims priorityunder 35 U.S.C. §120 to U.S. patent application Ser. No. 13/849,125,entitled “SINTER BONDED CONTAINMENT TUBE”, by Banach et al., filed Mar.22, 2013, which claims priority under 35 U.S.C. §119(e) to U.S. PatentApplication No. 61/614,508 entitled “SINTER BONDED CONTAINMENT TUBE,” byBanach et al., filed Mar. 22, 2012, both of which are assigned to thecurrent assignee hereof and incorporated herein by reference in theirentireties.

FIELD OF THE DISCLOSURE

This disclosure, in general, relates to containment tubes, and moreparticularly to sealed containment tubes comprising silicon carbide.

BRIEF DESCRIPTION OF THE INVENTION

A sintered ceramic sealed tube has first and second ends and an innerbore extending along at least a portion of its axial length between thefirst and second ends, the first end having a plug residing in the innerbore to close the first end, the second end having a distal wall toclose the inner bore at the second end. The ceramic tube, or the plug,or both, may comprise silicon carbide, and in certain embodimentscomprise principally silicon carbide, such that silicon carbide is themajority compositional species of the tube. The ceramic sealed tubeincludes a sinter bond between at the tube and at least the distal wallor the plug, such that the sinter bond forms a hermetic seal, orinterference bond, that includes no bond materials.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 shows an embodiment of a containment tube having radioactivematerial contents within the tube, and a distal wall and a plug sealingthe tube, and in which at least one of the distal wall or plug aresinter-bonded to the tube.

FIG. 2 shows an embodiment of a containment tube having two plugs.

FIG. 3 shows an embodiment of a green tube exposed to partial sintering.

FIG. 4 shows an embodiment of a partially green, un-sintered tube havingradioactive material contents and a plug, in which a portion of theun-sintered tube circumventing the plug is exposed to sintering.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION

Embodiments of the present invention are generally drawn to containmenttubes and methods for forming containment tubes. In one embodiment, acontainment tube includes a sealed tube comprising silicon carbide, thesealed tube having a generally constant diameter along its axial lengthand containing a radioactive material. A “generally constant diameter”means that the outer diameter of the tube does not vary considerablyfrom a nominal or average diameter value. According to one embodiment,any measureable diameter variances do not exceed 15% of the nominaldiameter value, such as not greater than 10%, not great than 5%, notgreater than 4%, not greater than 3%, not greater than 2%, or notgreater than 1%. In one embodiment, to the naked eye, the containmenttube appears to be uniform and rectilinear.

In particular, in one embodiment, a sintered ceramic sealed tube hasfirst and second ends and an inner bore extending along at least aportion of its axial length between the first and second ends, the firstend having a plug residing in the inner bore to close the inner bore atthe first end, the second end having a distal wall to close the secondend. The ceramic tube, or the plug, or both, may comprise siliconcarbide, and in certain embodiments comprise principally siliconcarbide, such that silicon carbide is the majority compositional speciesof the tube, typically greater than at least about 70 wt %, such asgreater than at least about 80 wt %, or greater than at least about 90wt %. In another embodiment, the tube may comprise silicon carbide in anamount greater than at least about 91 wt %, at least about 95%, at leastabout 99%, at least about 95%, at least about 99.85 wt %. One particularform of silicon carbide is used according to certain embodiments, knownas HEXOLOY®-brand silicon carbide (manufactured by Saint-Gobain AdvancedCeramics Corporation of Worcester, Mass., USA), described in U.S. Pat.No. 4,179,299 incorporated herein by reference. Suitable siliconcarbides generally contain silicon carbide in an amount greater than atleast about 91 wt %, such as greater than at least about 99.85 wt %, upto about 5.0 wt % carbonized organic material, from at least about 0.15wt % to not greater than about 3.0 wt % boron, and up to about 1.0 wt %additional carbon. The “carbonized organic material” is free carbon oruncombined carbon produced in situ by the carbonization of the organicmaterial used as a raw material in the process of forming the ceramictube. The carbonizable organic materials that can be used in forming theceramic tube are well known in the art, and include but are not limitedto phenolic resin, coal tar pitch, polyphenylene, orpolymethylphenylene, and the like.

Sintered ceramic bodies of silicon carbide according to an embodimentmay be characterized by a predominantly equiaxed microstructure, meaningthe presence of grains having an aspect ratio of less than 3:1 (i.e.,the ratio of the maximum dimension of the grains of the crystalmicrostructure to the minimum dimension of the grains of the crystalmicrostructure is less than 3:1). Moreover, the silicon carbidecomprises at least about 95 wt %, such as at least about 99 wt %alpha-phase, non-cubic crystalline silicon carbide.

The density of silicon carbide according to an embodiment is at leastabout 2.40 g/cm³, such as at least about 2.90 g/cm³, or at least about3.05 g/cm³.

A better understanding of the embodiments of the present invention maybe better had with reference to the drawings. In particular, inconnection with FIG. 1, an embodiment of a containment tube isillustrated. As shown, the containment tube 10 has a generally elongatebody, which may be described or quantified in terms of aspect ratio,which is the ratio of length to outside diameter. With respect to therelationship of length (L) to outer diameter (DO) referred to herein asaspect ratio, generally the tube will have an aspect ratio of not lessthan about 10:1, such as not less than about 20:1, such as not less thanabout 30:1, or not less than about 40:1. Typically, the aspect ratio islimited, as extended length tubes are difficult to handle and fullysinter. Consequently, aspect ratios typically do not exceed 300:1.

The containment tube 10 includes a first end 14, and a second end 12.Along the inner bore 13 of the containment tube, is provided a plug 16at the first end, closing the first end hermetically. In an embodiment,the plug 16 closes the first end and provides a hermitic seal by way ofa sinter bond, or interference bond, between the plug 16 and the firstend 14 of the tube 10. The second end has a distal wall 15 which, inthis particular embodiment, is integrated into the outer wall formingthe outside diameter of the tube. In an embodiment, the distal wall 15is integrated into the outer wall by a sinter bond, or interferencebond. Radioactive material 18 is disposed within the tube, generallyremote from first end 14 and plug 16.

Turning to FIG. 2, another embodiment of a containment tube isillustrated. Containment tube 20 includes a first end 21, a second end22, a bore 23, and first and second plugs 24 and 25 respectivelyprovided to close the bore at first and second ends 21, 22,respectively. That should be clear, the containment tube 20 differs fromcontainment tube 10 in that containment tube 20 has a dual-plugstructure. Accordingly, at least one of the first end 21 and the secondend 22 of the containment tube 20 includes a sinter bond, orinterference bond, with the respective first or second plugs 24 and 25.

According to a particular feature, containment tubes according toembodiments herein may be formed through a multi-step sintering process.Turning to FIG. 3, a green containment tube 30 is shown, having an innerbore 33, an outer wall 34, first and second ends 31, 32 respectively,the second end 32 having a distal wall 35 closing the bore at the secondend 32. The green tube 30 may be formed of any one of knownmanufacturing techniques. Although various forming techniques can beutilized for fabrication of the tube, such as slip casting, isopressing,machining of large stock materials, and other forming techniques,extrusion may be used according to particular embodiments. Extrusionrepresents a cost-effective and desirable fabrication approach formaking multiple articles requiring tubes of varying lengths anddiameters.

Sintered ceramic bodies of silicon carbide according to an embodimentmay be characterized by the amount the bodies shrink from a green stateto a fully sintered state. For example, green ceramic bodies of siliconcarbide according to an embodiment may shrink more than about 10% fromtheir original size, more than about 12%, more than about 15%, more thanabout 17%, less than about 25%, less than about 20%, less than about17%, less than about 15% upon being fully sintered. In a particularembodiment, a green ceramic body of silicon carbide may shrinkapproximately 17% from its original size upon being fully sintered. Whencombining a pre-sintered first component, such as a plug or distal wall,with a green second component, such as an un-sintered portion of a tube,that circumvents the pre-sintered first component, the shrinkagerelationship, and the amount of interference bond, can be expressed asfollows.

ID_(t,FS)=OD_(p)−Δ,

where

ID_(t,FS) is the inside diameter (ID) of a fully sintered tube, OD_(p)is the outside diameter (OD) of the pre-sintered plug, and Δ is theintereference (tube undersizement). For example, a pre-sintered plug hasa bond surface, or outside diameter, of 2.0″ (i.e. OD_(t)=2.0). Aninterference bond of 5% (i.e. Δ=5%) of a second body, such as the tube,requires a fully sintered tube ID (ID_(t,Fs)) to be 0.10″ less than theOD_(p) (i.e. 2.0*5%=0.10), or 1.90″ (i.e. ID_(t,Fs)=OD_(p)−Δ, or1.90″=2.0″−0.10″). Thus, to attain a 5% interference of a fully sinteredtube on the pre-sintered plug, the green portion of the tube (i.e. theun-sintered portion of the tube) will be made to have a theoreticallyfully sintered inner diameter (if it were sintered by itself) of 1.90″.

Further, the ID of the green second component (i.e. the un-sinteredportion of the tube) can be expressed as follows.

ID_(t,FS)/(1−R _(s))=ID_(t),

where

ID_(t) is the inner diameter of the green second component, orun-sintered portion of the tube, and R_(s) is the shrinkage rate of thesecond component (expressed as a decimal). Thus, in accordance with theexample given above, and assuming the shrinkage rate of the secondcomponent is 17.0%, the inner diameter of the green portion of the tube(ID_(t)) can be calculated as 1.9÷(1−0.170)=2.289″.

Following appropriate shape formation (i.e. forming of the green ceramictube), the green tube 30 may be subjected to a machining operationduring which the outer surface of the ceramic tube is machined prior topre-sintering. Stated alternatively, this machining step is carried outin the green state, where the tube is in a state that allows easiermaterial removal than in the sintered state. Moreover, the machining maybe effective to reduce or even completely remove dimensional(out-of-roundness) or surface irregularities of the green tube. Forexample, in the case of extrusion, the green tube may havecharacteristic score lines extending partially or wholly along theentire length of the tube. Those score lines can inhibit the formationof a strong interfacial sinter bond, as well as a hermetic seal. In thecase of other formation technologies, machining may still be desirable.For example, in the case of isopressing or molding, characteristicimperfections may be left behind on the green tube, such as a flashing.

Both the surface cleaning and machining steps may be carried out throughmechanical abrasion processes. Mechanical abrasion can include machiningusing a free abrasive (e.g., an abrasive slurry), a coated abrasive, ora fixed abrasive. The species of abrasive product is chosen to preventunwanted chemical interaction with or foreign deposits on the tube,while also providing adequate material removal rates. Generally speakingin the case of silicon carbide, abrasive materials such as alumina areavoided, and materials such as silicon carbide and superabrasives,notably including cubic boron nitride (CBN) and diamond, are utilized.In the green state, machining may be carried out with silicon carbideand in the sintered state, surface cleaning may be done with siliconcarbide or a superabrasive species. In practice, embodiments have madeuse of coated abrasives, such as a silicon carbide, CBN, or diamondabrasive coated on a closed looped belt, mounted to a belt sander.

While the cleaning steps above set forth in connection with a tube,particularly the outer surface of the tube, the foregoing cleaningoperations can be carried out with respect to an inner surface of thetube and particularly the plugs.

According to one particular feature, processing may continue withpartial sintering of the tube. As shown in FIG. 3, partial sintering 36is carried out with respect to a portion of the tube including a middleportion extending to the second end, leaving the first end unheated, oronly partially sintered such that the second end does not shrink to itsfinal dimensions. Upon completion of partial sintering 36, the tube isin a hybrid state, a central portion of the tube extending to the secondend being fully sintered and shrunk to its final dimensions, the firstend being still in the green state or only partially sintered. Thishybrid state can be better seen in connection with FIG. 4, in whichcontainment tube 40 shows sintered portion 42, and partially orun-sintered portion 44 extending to the first end 46. As can be seen,the first end 46 has a larger inside diameter and outside diameterrelative to the second end 48 which has already undergone densificationand shrinkage. Typically, the differences in outside diameter from thefirst end to the second end are on the order of 1-20%, sometimes larger.

Processing continues with incorporation with radioactive material 41into the inner bore 43 of the tube, and placement of plug 45 in the boreat the first end 46. Typically, the plug used to seal one or both endsof the containment tube structure is formed of the same material as thetube, in accordance with the above description.

Upon completion of forming a green ceramic plug, the plug proceeds to apre-sintering step to form a sintered plug. Pre-sintering can be carriedout in any one of known furnaces, including continuous furnaces or abatch furnace that translate the work piece (herein, the plug) throughthe furnace at a constant or variable rate. Pre-sintering is generallycarried out at a temperature above 2000° C., such as above 2050° C., butgenerally below 2400° C., such as below 2300° C., such as below 2250° C.A suitable target range for sintering the green ceramic plug in the caseof silicon carbide can lie within a range of 2100-2200° C. Sinteringtimes can vary, and are largely dependent on the thermal mass of theplug. However, typically sintering times range from 15 minutes to 10hours, such as not less than about 30 minutes, such as not less thanabout 1 hour, such as not less than about 1.5 hours. While large, highmass plugs may require extended sintering times, typically sinteringtimes do not exceed 30 hours, such as not great than 20 hours, such asnot greater than 10 hours.

Accordingly, the machining operations applied the tube can also beapplied to the outside of the plug. After the sintering step iscompleted, at least a portion of an outer surface of the sintered plugis subjected to surface cleaning. Typically, at least the portion of theplug that will contact the base component will be subjected to surfacecleaning. In this respect, it has been found that the outer surface ofthe plug can carry contaminates, such as contaminates that are depositedduring the sintering process, or which form as a consequence of thesintering process and changes in the crystallographic and compositionalstructure of the plug. For example, binders within the composition mayburn-out, leaving behind a carbonaceous residue on the outer surface ofthe plug. That carbonaceous residue, generally in the form of freecarbon, can negatively impact the quality of bond between the plug andthe base component, inhibiting a hermetic seal.

Thereafter, sintering 47 (i.e. co-sintering the first end 46 and theplug 45) takes place to complete the sintering of the partially orun-sintered portion 44, shrinking it to its final dimensions, forming acompleted containment tube having a generally constant outside diameter,and resembling the structure shown in FIG. 1, and providing a sinterbond between the first end 46 and the plug 45. The sinter bond betweenthe first end and the plug is defined as an interface bond, or aninterference bond, and includes no bond materials. The green siliconcarbide material of the un-sintered portion 44 of the first end 46shrinks to some degree upon sintering, and the quality of theinterference bond is at least in part due to selecting a size of thegreen, un-sintered second portion 44. As discussed above, the quality ofthe interference bond is also attributable to preparing the surface ofthe pre-sintered silicon carbide component to remove contaminants fromits surface.

The interface bond has at least one of the following performancecharacteristics: a Shear Strength not less than about 25 MPa, a NitrogenSeal Performance of not greater than 10%, a Helium Seal Performance ofnot greater than 10%, and/or a Vacuum Seal Performance of not greaterthan 10%.

In one embodiment, the interface between the first and the secondcomponent exhibits a Shear Strength not less than about 25 MPa, not lessthan about 40 MPa, not less than about 50 MPa, not less than about 75MPa, not less than about 100 MPa, not less than about 120 MPa, not lessthan about 140 MPa, not less than about 170 MPa, or not less than about200 MPa. In one embodiment, the interface between the first and thesecond component exhibits a Shear Strength not greater than about 1000MPa, such as not greater than about 700 MPa, not greater than about 500MPa, or not greater than about 300 MPa.

As used herein, reference to Shear Strength as a particular ShearStrength value is measured by testing a sample having standardizeddimensions under load. In particular, the Shear Strength is measured bypreparing and testing a standardized sample as follows. The sample isprepared from a ceramic tube and a ceramic ring, each having a length of76.2 mm The ceramic tube has an outer diameter (OD_(t)) of 14 mm and aninner diameter (ID_(t)) of 11 mm The ceramic ring has an outer diameter(OD_(t)) of 20 mm, and an inner diameter (ID_(r)) of 14 mm. The ceramicring is placed around the ceramic tube so that the ends of each areflush, and the tube-ring assembly is then co-sintered. After cooling, across-sectional center segment is sliced from the sintered assembly andthickness grinded to a final thickness (t) of 3 mm. The center segmentcomprises an inner ring sliced from the ceramic tube and an outer ringsliced from the ceramic ring. The area of contact between the inner andouter rings represents the total bond area (A_(b)), and is calculatedaccording to the following formula:

A _(t)=π·OD_(τ)·t  (Formula I)

The Shear Strength of the center segment sample is tested at roomtemperature using an Instron 8562 using a 100 kN load cell at a speed of0.05 mm/min, which applies equal but opposing force to the inner andouter rings, respectively. The magnitude of the applied force isgradually increased until the rings break apart. The force (F) requiredto break the rings apart is measured in Newtons. The Shear Strength (τ)value is obtained according to the following formula:

τ=F·A _(b)·10^(t)  (Formula II)

It should be understood that ceramic articles as described herein can bea wide variety of dimensions and overall sizes, but the Shear Strengthvalues are based on a standardized geometry and testing approach asdescribed above. Consequently, validating the Shear Strength of a samplehaving differing dimensions larger or smaller than the standardizedsample described above requires the fabrication of a standardized sampleunder identical compositional and processing conditions to that of thesample having differing dimensions.

A Nitrogen Seal Performance is determined according to a nitrogen sealperformance test, wherein nitrogen is applied at an interface of a sealat a given initial positive pressure, and pressure loss is measured by apressure gauge. Nitrogen Seal Performance is the percent pressure dropoccurring across the seal interface over a period of 2 hours at anapplied gauge pressure, such as 200 psi. Embodiments herein achieve aNitrogen seal performance of not greater than 10%, not greater than 9%,not greater than 8%, not greater than 7%, not greater than 6%, notgreater than 5%, not greater than 4%, not greater than 3%, not greaterthan 2%, not greater than 1.9%, not greater than 1.8%, not greater than1.7%, not greater than 1.6%, not greater than 1.5%, not greater than1.4%, not greater than 1.3%, not greater than 1.2%, not greater than1.1%, not greater than 1.0%, not greater than 0.9%, not greater than0.8%, not greater than 0.7%, not greater than 0.6%, not greater than0.5%, not greater than 0.4%, not greater than 0.3%, not greater than0.2%, or not greater than 0.1% of an initial pressure differential of200 PSI (gauge pressure).

A Helium Seal Performance is determined according to a helium sealperformance test, wherein helium is applied at an interface of a seal ata given initial positive pressure and pressure loss is measured by apressure gauge. Helium Seal Performance is achieved if the pressure dropoccurring across the seal interface over a period of 2 hours is notgreater than 10%, not greater than 9%, not greater than 8%, not greaterthan 7%, not greater than 6%, not greater than 5%, not greater than 4%,not greater than 3%, not greater than 2%, not greater than 1.9%, notgreater than 1.8%, not greater than 1.7%, not greater than 1.6%, notgreater than 1.5%, not greater than 1.4%, not greater than 1.3%, notgreater than 1.2%, not greater than 1.1%, not greater than 1.0%, notgreater than 0.9%, not greater than 0.8%, not greater than 0.7%, notgreater than 0.6%, not greater than 0.5%, not greater than 0.4%, notgreater than 0.3%, not greater than 0.2%, or not greater than 0.1% of aninitial pressure differential of 87 PSI (gauge pressure), an initialpressure differential of about 200 psi (about 13.8 bar), or an initialpressure differential of about 6 barg (bar gauge).

A Vacuum Seal Performance is determined according to a vacuum sealperformance test. In the vacuum seal performance test, a vacuum isapplied to a seal. The nitrogen gas atmosphere inside the tube is thenlowered from 1 ATM (760 torr) to a pressure of 10 torr thereby having apressure differential of 750 torr. Vacuum Seal Performance is achievedif the gain inside the tube occurring across the seal interface over aperiod of 2 hours is not greater than 10%, not greater than 9%, notgreater than 8%, not greater than 7%, not greater than 6%, not greaterthan 5%, not greater than 4%, not greater than 3%, not greater than 2%,not greater than 1.9%, not greater than 1.8%, not greater than 1.7%, notgreater than 1.6%, not greater than 1.5%, not greater than 1.4%, notgreater than 1.3%, not greater than 1.2%, not greater than 1.1%, notgreater than 1.0%, not greater than 0.9%, not greater than 0.8%, notgreater than 0.7%, not greater than 0.6%, not greater than 0.5%, notgreater than 0.4%, not greater than 0.3%, not greater than 0.2%, or notgreater than 0.1% of the pressure differential (750 torr).

In each of the seal performance tests, the bond or interface issubjected to the above-described pressure differential. Depending on thegeometry of the part, an inner volume is pressurized or evacuated, andholes plugged. In a case of an external seal, such as in the case of aflange on a tube, an end-cap is positioned to cover the flange andexposed bore of the tube, the cap being offset from the bore to allowfluid communication (and hence pressure/vacuum) extending radially up tothe bond region. Caps/plugs can have varying geometries to fit the partundergoing test, and can be sealed with a vacuum grease to ensure apressure tight, hermetic seal.

What is claimed is:
 1. A containment tube comprising: a sintered tubecomprising silicon carbide; a generally constant diameter along an axiallength of the sintered tube; and a plug comprising silicon carbide,wherein the sintered tube is sealed.
 2. The containment tube of claim 1,the sintered tube, the plug, or both comprise at least 70 wt % ofsilicon carbide.
 3. The containment tube of claim 2, the sintered tube,the plug, or both comprise at least 90 wt % of silicon carbide.
 4. Thecontainment tube of claim 1, wherein the plug is sinter-bonded to thesintered tube.
 5. The containment tube of claim 1, wherein the sinteredtube has first and second ends and an inner bore extending along atleast a portion of its axial length between the first and second ends,the first end having the plug residing in the inner bore to close thefirst end, and the second end having a distal wall that closes the innerbore at the second end.
 6. The containment tube of claim 1, wherein thesintered tube contains a radioactive material.
 7. The containment tubeof claim 1, wherein the sintered tube has: a Shear Strength not lessthan about 25 MPa; or a Nitrogen Seal Performance of not greater than10%; or a Helium Seal Performance of not greater than 10%; or a VacuumSeal Performance of not greater than 10%; or any combination thereof. 8.A containment tube comprising: a tube comprising silicon carbide andhaving a first end, a closed second end, and an inner bore extendingbetween the first end and the second end; and a plug sinter-bonded to aninner wall proximate to the first end of the tube to close the firstend.
 9. The containment tube of claim 8, wherein the tube comprisingsilicon carbide is hermetically sealed.
 10. The containment tube ofclaim 8, wherein the second end has a distal wall integrated to an outerwall of the tube comprising silicon carbide.
 11. The containment tube ofclaim 8, wherein the tube comprising silicon carbide has a generallyconstant diameter along an axial length of the tube.
 12. The containmenttube of claim 8, wherein the tube comprises at least 70 wt % of siliconcarbide.
 13. The containment tube of claim 8, wherein the plug comprisesat least 70 wt % of silicon carbide.
 14. A method of forming acontainment tube comprising: providing a green ceramic tube having afirst end that is unsealed and a second end that is sealed; partiallysintering the green ceramic tube, leaving the first end unsintered orincompletely sintered; inserting a sintered plug into the first end; andsealing the first end by sintering the first end together with thesintered plug.
 15. The method of claim 14, further comprising formingthe green ceramic tube by extrusion.
 16. The method of claim 14, whereinpartially sintering comprises leaving the unsealed end incompletelysintered so that the first end of the tube is not shrunk to finaldimensions.
 17. The method of claim 14, wherein sealing the first endcomprises applying localized heat to the first end.
 18. The method ofclaim 14, further comprising placing a radioactive material into thepartially sintered tube via the first end prior to inserting a sinteredplug into the first end.
 19. The method of claim 14, wherein the ceramictube, the plug, or both comprises at least 70 wt % of silicon carbide.20. The method of claim 14, further comprising removing surfacecontaminants from an inner circumferential surface of the first end ofthe tube, an outer circumferential surface of the plug, or both.